The objectives of this study were to determine if viruses exist in deep granitic groundwater and to analyse their abundance and morphological diversity. Fluorescent microscopy counts on 10 groundwater samples ranging from 69 to 450 m depth were in the range of 10(4)-10(6) TNC ml(-1) (TNC, total number of prokaryotic cells) and 10(5)-10(7) VLP ml(-1) (VLP, virus-like particles). A good positive correlation of VLP with TNC (r=0.91, P=0.0003) was found with an average VLP/TNC ratio of 12. Transmission electron microscopy revealed four distinct bacteriophage groups (polyhedral, tailed, filamentous and pleomorphic) with at least seven phage families of which some are known to be lytic. Our results suggest the presence of viruses in deep granitic groundwater up to 450 m depth. If they are active and lytic, they will constitute an important group of predators that might control the numbers of microorganisms in the analysed groundwater.